1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) filter which is installed on a vehicular terminal such as a cellular phone or the like and distinguishes a high frequency signal, and especially relates to a technical field of a surface acoustic wave filter which performs unbalanced balanced conversion of an input-output signal and makes it to a lower impedance.
2. Description of the Related
FIG. 5 shows an example of installing a surface acoustic wave filter on a front end portion of a vehicular terminal. In the present example, an antenna switch 2 is disposed immediately below an antenna 1, and on a reception side, the surface acoustic wave filter distinguishes received high frequency signals coming through the antenna switch 2 by a receive SAW filter unit 3 formed on a piezoelectric substrate. On a transmitting side, it outputs transmitting high frequency signals distinguished by a transmission SAW filter unit 4 to the antenna 1 via the antenna switch 2.
A reception output of the SAW filter unit 3 is to be an input of a receive amplifier (LNA: Low Noise Amplifier) 6 obtaining impedance matching by a matching circuit 5. Similarly, an output of a transmission amplifier (PA: Power Amplifier) 7 is to be an input of the SAW filter unit 4 obtaining impedance matching through a matching circuit 8.
The antenna switch 2 takes an unbalanced input-output circuit structure, whereas the receive amplifier 6 takes a balanced input circuit structure (for instance, a differential amplifying circuit). Therefore, the SAW filter unit 3 is made into a circuit structure to obtain a balanced output from an unbalanced input, the transmission amplifier 7 takes a balanced output circuit structure (for instance, high frequency transformer output). Accordingly, the SAW filter unit 4 is made into a circuit structure to obtain an unbalanced output from a balanced input.
As a circuit system enabling the unbalanced balanced conversion, there is a structure such that two 3IDT-LCMRs (3IDT type longitudinal Coupled mode resonator type SAW filter unit) which are 180 degrees different in input phase from each other, are electrically connected in parallel to serve as an input terminal, and a balanced terminal output is formed by electrically connecting the 3IDT-LCMRs in series to be an output, so that a balanced driving output having a high terminal impedance is obtained by using this series connection point as a ground lug (for instance, see Patent Document 1).
It should be noted that the 3IDT-LCMR is prepared by forming three IDTs (Inter Digital Transducer) 91 to 93 having a plurality of electrode fingers in the direction of propagation of the surface acoustic wave on a piezoelectric substrate, and by further providing reflectors 94 and 95 at the outermost thereof. As for the IDT 91 (93), a pair is formed by disposing comb electrodes 91a (93a) and 91b (93b) so as to engage with each other. The comb electrode 91a (93a), which is one of the pair, is connected to, for instance, an input port 96, and the comb electrode 91b (93b), which is the other of the pair, is grounded, for instance. Further, as for the IDT 92, a pair is formed by disposing comb electrodes 92a and 92b so as to engage with each other, and the one is grounded while the other is connected to, for instance, an output port 97.
In the 3IDT-LCMR thus structured, an electric signal inputted into one input side IDT (for instance, 91, 93) is converted into a surface acoustic wave to propagate to the other output side IDT (for instance, 92), so that the surface acoustic wave is taken out by being reversely converted into an electric signal on this output side IDT. Thus, the output phase and the input phase can be reversed by 180 degrees according to the difference between the electrode arrangement of the input side IDT and the electrode arrangement of the output side IDT. Furthermore, it is possible to modify the propagation characteristics of the surface acoustic wave, that is the filter frequency characteristic or the like according to the difference in pitch, shape, or the like of electrode fingers forming the comb electrode of the IDT.
As another circuit system which can carry out unbalanced balanced conversion, there is a technology such that a balanced type surface acoustic wave filter connecting both a series arm surface acoustic wave resonator and a parallel arm surface acoustic wave resonator in a symmetrical lattice shape is connected between a pair of electrodes of the output side IDT or the input side IDT of the SAW filter unit (for instance, see Patent Document 2).
[Patent Document 1] Japanese Patent Application Laid-open No. Hei 10-117123
[Patent Document 2] Japanese Patent Application Laid-open No. Hei 8-65098
Recently, miniaturization and lower pricing of vehicular terminals have progressed, and reduction in the number of parts of the high frequency units is an important subject. Furthermore, demand for reduction of the terminal impedance of a receive amplifier disposed at the rear stage of an SAW filter unit, from conventional high impedance of 100 Ω to several hundreds Ω to low impedance of less than 50 Ω, has also increased.
For instance, in a configuration shown in FIG. 5, a signal inputted from an antenna 1 is transmitted to a receive SAW filter unit 3 at required impedance of 50 Ω terminal impedance in an unbalanced driving by a switch 2 on the reception side, and a receive amplifier 6 in a rear stage of the SAW filter unit 3 has been required to have a terminal impedance of as low as about 30 Ω to 50 Ω in a balanced driving, in recent years. Accordingly, in the SAW filter unit 3 disposed in a front stage of the receive amplifier 6, it has been demanded that the input be 50 Ω at the terminal end in an unbalanced driving, and the output be about 30 Ω to 50 Ω in a balanced driving, and a matching circuit 5 which has been an indispensable circuit can be eliminated by realizing such an input and output impedance.
When an SAW filter unit, the terminal impedance of which is a high balanced driving output, is used to cope with these required electric performances, as in Patent Document 1, matching circuits 5 and 8 are required to be disposed between the receive amplifier 6 and a transmission amplifier 7, and the number of parts (capacitors and coils) for the matching circuits, which requires a large circuit space. Furthermore, designing and adjustment of the matching circuit becomes complicated.
Concerning to this point, in the system disclosed in Patent Document 2, the output impedance can be kept low. However, the out-of-band damping characteristic, which is an important characteristic for the SAW filter unit, is not sufficient.